(1) Field of Invention
This invention relates to communication systems and methods of operation. More particularly, the invention relates to adaptive power management based on a Rake Receiver configuration in WCDMA systems and methods of operation.
(2) Description of the Prior Art
In wireless communication system, signal fading due to multipath radio propagation severely degrades the performance and imposes high transmitter power requirement. Since the characteristic of a channel changes rapidly, a transmitter and a receiver can not be configured to operate at their optimum performance level and therefore, they fail to exploit full potential of the wireless system. Code-Division Multiple Access (CDMA) provides increased capacity due to the fact that each user in this system occupies the entire frequency band and therefore there is no waste of bandwidth due to channel spacing. Several systems have been proposed for the third generation wireless system. The most popular system under study is the Wideband CDMA (W-CDMA) system, described in an article entitled xe2x80x9cChannel Estimation for the W-CDMA System, Performance and Robustness Analyses from a Terminal Perspective,xe2x80x9d by B. Lindof, C. Ostberg, and H. Eriksson, published at the IEEE Vehicular Technology Conference. Document 90. May 1999.
Developers of the third generation wireless system in the industry envision crystal clear voice service, video conferencing from anywhere, high speed mobile Web surfing, and thousands of advanced applications right over the wireless phone or handheld PC. Generally, any enhancement to the system that can improve delivery of high-speed data, voice and video over mobile devices along with increasing the battery life are challenging topics for consideration and improvement.
In papers entitled xe2x80x9cSymbol Rate and Modulation Level-Controlled Adaptive Modulation/TDMA/TDD System for High-Bit-Rate Wireless Data Transmission,xe2x80x9d by T.Ue, S. Sampei, N. Morinaga, and K. Hamaguchi, published in the IEEE Transaction on Vehicular Technology, Vol. 47. No. 4, Pp. 1134-1147, November 1998, pages 1134-1147, and xe2x80x9cAdaptive Coding and Processing Gain Control with Channel Activation for Multimedia DS/CDMA System,xe2x80x9d by S. Abeta, S. Sampei, and N. Morinaga, published in IEICE Transaction on Communication, Vol. E80-B, No. 4. April 1997, pages    the authors propose a symbol rate, gain and coding change scheme through the use of feedback transmission of the information from the Base Station (BS) to the Mobile Station (MS). In these proposals the quality of the channel was determined on the basis of the calculation of the short-term signal to interference ratio C/(N0+I0) at the BS receiver, where C is Signal Power; N0 is AWGN Power, and I0 is Interference from other users. However, in a wideband environment, due to the presence of Inter-Symbol Interference (ISI), the short term Signal To Noise Ratio (SNR) is inadequate for measuring the quality of the channel, as described in an article entitled xe2x80x9cUpper-bound Performance of a Wideband Burst-by-Burst Adaptive Modem,xe2x80x9d by C. H. Wong, and L. Hanzo, published in the IEEE Vehicular Technology Conference. Document 483. May 1999, pages    .
In these adaptive schemes, thresholds are set based on the probability distribution function (pdf) of the channel power. In a 1-user model with one channel path, the pdf of the channel power would be an exponential or chi-square function with 2 degrees of freedom. However, in a CDMA receiver, normally the rake receiver has several fingers. That is at the receiver, the system either estimates or predicts the channel coefficients at each rake finger and performs maximal ratio combining by multiplying each finger with its conjugate or chooses the ones with the highest energy and performs maximal ratio combining on the selected fingers. In either case system performs the long-range power prediction of each finger at the transmitter to compute the total channel power.
Other prior art related to WCDMA systems with improved performance include:
U.S. Pat. No. 5,822,381 to E. Tiedemann, Jr. et al., issued Oct. 13, 1998 (Tiedemann) discloses a method and apparatus for controlling transmission power in a variable rate communication system. The method disclosed provides for a closed loop power control method. A first remote station controls the transmission power of a second remote station by transmitting a rate dependent power control signal to the second remote communications system. Since only the second communications system knows its transmission rate a priori, it must determine a course of action in accordance with both the received power control signal and the knowledge of its transmission rate.
U.S. Pat. No. 5,715,526 to L. A. Weaver, Jr., et al., issued Feb. 3, 1998, (Weaver) discloses an apparatus and method for controlling a final transmit power, Y of a base station in a cellular communications system that has several channels. The base station has a transmit power tracking gain, Yxe2x80x2 and a radio frequency transmit power, W. The apparatus comprises channel elements for calculating expected power Pk,axe2x88x92Pkf, each of which corresponds to a channel. The apparatus also comprises a transceiver system controller (BTSC) for generating a desired output power, Yd of the base station, including an adder for summing the expected powers. The apparatus also includes a transmit power detector for measuring xe2x80x98Yxe2x80x99 to obtain the measured transmit power. The apparatus further comprises a radio frequency interface card (RFIC) for generating xe2x80x98Yxe2x80x99. Finally, the apparatus includes a gain unit for processing xe2x80x98Yxe2x80x99 and W to obtain the final transmitted power, Y.
U.S. Pat. No. 5,383,219 to C. E. Wheatley, III, et al., (Wheatley) issued Jan. 17, 1995, discloses a power control process which enables a mobile radio telephone to continuously update the base station on the power output required. The base station sends a frame to the mobile at a particular rate. If the mobile received and decoded the frame correctly, the mobile sets a power control bit and the next frame to be transmitted to the base station. Based on the error rate of the received power control bits, the base station determines whether to increase or decrease the transmit power.
U.S. Pat. No. 5,729,557 to S. H. Gardner, et al, issued Mar. 17, 1998, (Gardner) discloses a method and apparatus for using multiple code rates for forward error correction in a cellular digital radio communication system. Each base station broadcasts a quantity called the power product (PP) which is equal to the base transmit power, PBT multiplied by the power level received at the base station, PBR. For a mobile-unit to determine it""s appropriate transmit power, PMT requires measuring the power received, PMR at the mobile unit and performing a calculation. When the channel path loss is large it is possible that the power control calculation will return a value greater than the maximum transmit power capability of the mobile unit. In such case, the mobile unit selects a lower code rate. The base station receiver sensitivity improves as the code rate decreases, so the result is similar to increasing the transmit power. In the preferred embodiment, the invention uses three different code rates. In most cases, the code rate used is two-thirds, but when a mobile unit determines that it needs more transmit power than it is capable of providing, the code range is changed to one-half, and in severe cases the code rate is changed to one-third.
JP6-276176 to Tetsuyoshi et al, published Sep. 30, 1994 (Tetsuyoshi) discloses reducing intra-signal interference at the time of demodulating signals from respective remote stations by preparing plural chip rates and appropriately allocating them for the respective remote stations. When the power level of reception signals initially detected by reception power detection or the signals from remote stations, a chip rate deciding circuit judges that the reception power level causes strong interference and the inverse spread demodulation of the signals. The present chip rate in this case is changed and the remote station is informed from a chip rate informing circuit. In a remote station a spreading code is generated corresponding to the chip rate informed from the base station. A spreading code is generated supplied to a spectrum spread modulation part to perform spread spectrum spread modulation and transmitted to the base station. Thus, the base station performs an inverse spread processing by the chip rate and interference is reduced at the remote stations.
In prior art systems, past estimates of the signal to interference ratio are used to adjust the transmitter power. Due to the fading of the wireless channels, past estimates of the received SNR is not an adequate technique for optimum power control. None of the prior art uses future prediction of the channel power by calculating a long-range prediction of each finger of a rake receiver and based on the estimated total channel power distribution function to set the optimum threshold to control transmitter gain and rate as in the present invention.
An object of the invention is a communication system such as a WCDMA system and method of operation having adaptive modulation for improved system throughput, channel capacity and transmit power control.
Another object is a WCDMA system and method of operation with improved adaptive power management using power levels of a Rake Receiver configuration.
Another object is a WCDMA system and method of operation which predicts power levels of each finger in a Rake receiver and the strongest power levels being used in determining the optimum transmitter power or rate control for operating the system transmitters and receivers.
These and other objects, features and advantages are achieved in a WCDMA system and method which maximizes throughput, control channel capacity/transmit power and maintains connectivity between a base and a mobile station using the predicted power levels of each finger in a rake receiver with the strongest ones used in determining the optimum transmitter power or rate control for operating the system tranmitters and receivers. The WCDMA system includes a Base Station (BS) or forward transmitter and a pilot channel that transmits control signals between a Mobile Station (MS) and BS to reconfigure their transmitter/receiver according to the prediction of the channel power and channel power probability density function separated into three distinct equal probable regions. Data signals are encoded using a one-half Viterbi encoder and interleaved. The interleaved data bits are modulated using Quadrature Phase Shift Keying (QPSK) modulation. The QPSK data is multiplexed with the pilot channel and spread by an appropriate code in an OFDM transmitter modified by a long code. Output of the transmitter may be provided to two diverse antennas for reliable communications to the receiver. Data may be received at two diverse antennas. The outputs are provided to match filters coupled to a coherent rake receiver and a channel prediction system. The future attenuation of the channel coefficients and power are determined by the prediction system for several milliseconds. The power levels of each finger in the Rake receiver can be predicted and the strongest ones used in determining the optimum transmitter power or rate control for operating the system transmitters and receivers based on computing a long range power prediction of each finger of the rake receiver.